Production of Superabsorbent Polymers on a Continuous Belt Reactor

ABSTRACT

The invention relates to the production of superabsorbent polymers on a continuous belt reactor, comprising at least one rotating knife that cuts the formed polymer gel at the end of the continuous belt reactor, wherein the length of the cutting edge is at least 1 cm and the cutting edge is non-parallel to the rotation axis.

The present invention relates to the production of superabsorbentpolymers on a continuous belt reactor, comprising at least one rotatingknife that cuts the formed polymer gel at the end of the continuous beltreactor, wherein the length of the cutting edge is at least 1 cm and thecutting edge is non-parallel to the rotation axis.

Superabsorbent polymers are in particular polymers of (co)polymerizedhydrophilic monomers, graft (co)polymers of one or more hydrophilicmonomers on a suitable grafting base, crosslinked ethers of cellulose orof starch, crosslinked carboxymethylcellulose, partially crosslinkedpolyalkylene oxide or natural products swellable in aqueous fluids, suchas guar derivatives for example. Such polymers are used as productscapable of absorbing aqueous solutions to produce diapers, tampons,sanitary napkins and other hygiene articles, but also as water-retainingagents in market gardening.

Superabsorbent polymers typically have a Centrifuge Retention Capacityin the range from 25 to 60 g/g, preferably of at least 30 g/g, morepreferably of at least 32 g/g, even more preferably of at least 34 g/gand most preferably of at least 35 g/g. Centrifuge Retention Capacity(CRC) is determined by EDANA (European Disposables and Non-wovensAssociation) recommended test method No. WSP 241.2-05 “Centrifugeretention capacity”.

To improve their performance characteristics, for example permeability,superabsorbent polymeric particles are generally postcrosslinked. Thispostcrosslinking can be carried out in the aqueous gel phase.Preferably, however, dried, ground and screened particles of the basepolymer are surface coated with a postcrosslinker, dried and thermallypostcrosslinked. Useful crosslinkers for this purpose include compoundscomprising at least two groups capable of forming covalent bonds withthe carboxylate groups of the superabsorbent polymer particles orcapable of crosslinking together carboxyl groups or other functionalgroups of at least two different polymeric chains of the base polymer.

The production of superabsorbent polymers is described for example inthe monograph “Modern Superabsorbent Polymer Technology”, F. L. Buchholzand A. T. Graham, Wiley-VCH, 1998, pages 69 to 117.

Kneading reactors or belt reactors are suitable reactors. In a kneader,the polymer gel which is produced in the course of the polymerization ofan aqueous monomer solution is for example continuously comminuted bycontrarotatory stirring shafts, as described in WO 2001/38402 A1. Thepolymerization on a belt is described for example in DE 38 25 366 A1 andU.S. Pat. No. 6,241,928. The polymerization in a belt reactor produces apolymer gel which has to be comminuted in a further process step, forexample in a meat grinder, extruder or kneader.

JP 11188725 A discloses a vertical cutter for crushing water-containinggels.

It is an object of the present invention to provide an improved processfor production of superabsorbent polymers on a continuous belt.

We have found that this object is achieved by a process for productionof superabsorbent polymers on a continuous belt reactor, comprising atleast one rotating knife that cuts the formed polymer gel at the end ofthe continuous belt reactor, wherein the length of the cutting edge isat least 1 cm and the cutting edge is non-parallel to the rotation axis.

Non-parallel means a deviation of up to preferably at least 5 degree,more preferably at least 15 degree, most preferably at least 20 degree.

The length of the cutting edge is preferably at least 5 cm, morepreferably at least 20 cm, most preferably at least 100 cm.

The change of the force that rests on the rotation axis during onerevolution is preferably less than 20%, more preferably less than 10%,most preferably less than 5%. The change of force can be measured by thepower input of the rotating knife. The change of force is dependent onthe change of the cutting length per unit of time and can be adjusted bychanging the inclination of the cutting edges, the number of knifes andthe length of the cutting edges.

In a preferred embodiment of the present invention the at least onerotating knife is at least one rotating spiral knife. A rotating spiralknife is a cutting means that is mounted in a helical manner on aroll-like device.

FIG. 1 is a schematic view of a rotating spiral knife used in theprocess of the invention.

FIG. 2 is a cross-sectional view of a rotating spiral knife used in theprocess of the invention.

FIG. 3 is a detailed cross-sectional view of rotating spiral knife usedin the process of the invention that shows the construction forreinforcement of the knifes.

An effect of the invention is that the rotating knife have a highlyimproved serviceable life compared to the rotating knifes of the priorart. Thus, the invention reduces the maintenance costs of the productionprocess.

In a preferred embodiment of the present invention the polymer gel movesdownward at the end of the continuous belt reactor and the rotatingknife cuts the downward moving polymer gel.

Preferably, the cut polymer gel is further disintegrated in an extruder.The extruder can be placed under the rotating knife that the cut polymergel falls directly into the extruder.

The difference of the speed of the continuous belt and the tip speed ofthe rotating knife is preferably less than 25%, more preferably lessthan 10%, most preferably less than 5%.

In another preferred embodiment of the present invention the end of oneknife in rotating direction is the beginning of the same or the nextknife at the other lateral end of the polymer gel to be cut. That meansthat the next cut starts at the time when the previous cut ends. FIG. 1shows an example of this embodiment.

In another preferred embodiment of the present invention the roll-likedevice is a cylinder, wherein the diameter of the cylinder including theknife is preferably from 20 cm to 70 cm, more preferably from 30 to 60cm, most preferably from 35 to 55 cm, and diameter of the cylinderexcluding the knife is preferably from 10 cm to 60 cm, more preferablyfrom 20 to 50 cm, most preferably from 25 to 45 cm.

Preferably, n spiral knifes are mounted on a cylinder and each spiralknife forms a 1/n turn spiral wherein n is an integer, i.e. 2, 3, 4, 5and 6. More preferably n is 4 as shown in FIG. 1.

The monomer solutions or monomer suspensions usable in the process ofthe present invention comprises

-   a) at least one ethylenically unsaturated acid-functional monomer,-   b) at least one crosslinker,-   c) if appropriate one or more ethylenically and/or allylically    unsaturated monomers copolymerizable with a), and-   d) if appropriate one or more water-soluble polymers onto which the    monomers a), b) and if appropriate c) can be at least partly    grafted.

Suitable monomers a) are for example ethylenically unsaturatedcarboxylic acids, such as acrylic acid, methacrylic acid, maleic acid,fumaric acid and itaconic acid and/or salts of these acids. Acrylic acidand methacrylic acid are particularly preferred monomers. Acrylic acidis most preferable.

Useful monomers a) are further styrenesulfonic acid,2-acrylamido-2-methylpropanesulfonic acid and 2-hydroxyethylacrylate.

The proportion of the total amount of monomers a) which is attributableto acrylic acid and/or its salts is preferably at least 50 mol-%, morepreferably at least 90 mol-% and most preferably at least 95 mol-%.

The monomers a) and especially acrylic acid comprise preferably up to0.025% by weight of a hydroquinone half ether. Preferred hydroquinonehalf ethers are hydroquinone monomethyl ether (MEHQ) and/or tocopherols.

Tocopherol refers to compounds of the following formula:

where R¹ is hydrogen or methyl, R² is hydrogen or methyl, R³ is hydrogenor methyl and R⁴ is hydrogen or an acid radical of 1 to 20 carbon atoms.

Preferred R⁴ radicals are acetyl, ascorbyl, succinyl, nicotinyl andother physiologically tolerable carboxylic acids. The carboxylic acidscan be mono-, di- or tricarboxylic acids.

Preference is given to alpha-tocopherol where R¹=R²=R³=methyl,especially racemic alpha-tocopherol. R⁴ is more preferably hydrogen oracetyl. RRR-alpha-Tocopherol is preferred in particular.

The monomer solution comprises preferably not more than 130 weight ppm,more preferably not more than 70 weight ppm, preferably not less than 10weight ppm, more preferably not less than 30 weight ppm and especiallyabout 50 weight ppm of hydroquinone half ether, all based on acrylicacid, with acrylic acid salts being arithmetically counted as acrylicacid. For example, the monomer solution can be produced using an acrylicacid having an appropriate hydroquinone half ether content.

The superabsorbent polymers are in a crosslinked state, i.e., thepolymerization is carried out in the presence of compounds having two ormore polymerizable groups which can be free-radically interpolymerizedinto the polymer network. Useful crosslinkers b) include for exampleethylene glycol dimethacrylate, diethylene glycol diacrylate, allylmethacrylate, trimethylolpropane triacrylate, triallylamine,tetraallyloxyethane as described in EP 530 438 A1, di- and triacrylatesas described in EP 547 847 A1, EP 559 476 A1, EP 632 068 A1, WO 93/21237A1, WO 2003/104299 A1, WO 2003/104300 A1, WO 2003/104301 A1 and DE 10331 450 A1, mixed acrylates which, as well as acrylate groups, comprisefurther ethylenically unsaturated groups, as described in DE 103 31 456A1 and DE 103 55 401 A1, or crosslinker mixtures as described forexample in DE 195 43 368 A1, DE 196 46 484 A1, WO 90/15830 A1 and WO2002/32962 A2.

Useful crosslinkers b) include in particular N,N′-methylenebisacrylamideand N,N′-methylenebismethacrylamide, esters of unsaturated mono- orpolycarboxylic acids of polyols, such as diacrylate or triacrylate, forexample butanediol diacrylate, butanediol dimethacrylate, ethyleneglycol diacrylate, ethylene glycol dimethacrylate and alsotrimethylolpropane triacrylate and allyl compounds, such as allyl(meth)acrylate, triallyl cyanurate, diallyl maleate, polyallyl esters,tetraallyloxyethane, triallylamine, tetraallylethylenediamine, allylesters of phosphoric acid and also vinylphosphonic acid derivatives asdescribed for example in EP 343 427 A2. Useful crosslinkers b) furtherinclude pentaerythritol diallyl ether, pentaerythritol triallyl ether,pentaerythritol tetraallyl ether, polyethylene glycol diallyl ether,ethylene glycol diallyl ether, glycerol diallyl ether, glycerol Wallylether, polyallyl ethers based on sorbitol, and also ethoxylated variantsthereof. The process of the present invention utilizes di(meth)acrylatesof polyethylene glycols, the polyethylene glycol used having a molecularweight between 300 and 1000.

However, particularly advantageous crosslinkers b) are di- andtriacrylates of 3- to 20-tuply ethoxylated glycerol, of 3- to 20-tuplyethoxylated trimethylolpropane, of 3- to 20-tuply ethoxylatedtrimethylolethane, especially di- and triacrylates of 2- to 6-tuplyethoxylated glycerol or of 2- to 6-tuply ethoxylated trimethylolpropane,of 3-tuply propoxylated glycerol, of 3-tuply propoxylatedtrimethylolpropane, and also of 3-tuply mixedly ethoxylated orpropoxylated glycerol, of 3-tuply mixedly ethoxylated or propoxylatedtrimethylolpropane, of 15-tuply ethoxylated glycerol, of 15-tuplyethoxylated trimethylolpropane, of at least 40-tuply ethoxylatedglycerol, of at least 40-tuply ethoxylated trimethylolethane and also ofat least 40-tuply ethoxylated trimethylolpropane.

Very particularly preferred for use as crosslinkers b) are diacrylated,dimethacrylated, triacrylated or trimethacrylated multiply ethoxylatedand/or propoxylated glycerols as described for example in WO 2003/104301A1. Di- and/or triacrylates of 3- to 10-tuply ethoxylated glycerol areparticularly advantageous. Very particular preference is given to di- ortriacrylates of 1- to 5-tuply ethoxylated and/or propoxylated glycerol.The triacrylates of 3- to 5-tuply ethoxylated and/or propoxylatedglycerol are most preferred. These are notable for particularly lowresidual levels (typically below 10 weight ppm) in the water-absorbingpolymer and the aqueous extracts of water-absorbing polymers producedtherewith have an almost unchanged surface tension compared with waterat the same temperature (typically not less than 0.068 N/m).

The amount of crosslinker b) is preferably from 0.001 to 10 wt. %, morepreferably from 0.01 to 5 wt. % and most preferably from 0.1 to 2 wt. %,all based on monomer a).

Examples of ethylenically unsaturated monomers c) which arecopolymerizable with the monomers a) are acrylamide, methacrylamide,crotonamide, dimethylaminoethyl methacrylate, dimethylaminoethylacrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate,dimethylaminobutyl acrylate, dimethylaminoethyl methacrylate,diethylaminoethyl methacrylate, dimethylaminoneopentyl acrylate anddimethylaminoneopentyl methacrylate.

Useful water-soluble polymers d) include polyvinyl alcohol,polyvinylpyrrolidone, starch, starch derivatives, polyglycols orpolyacrylic acids, preferably polyvinyl alcohol and starch.

The solids content of the monomer solution is preferably at least 30 wt.%, more preferably at least 35 wt. %, most preferably at least 40 wt. %.The solids content is the sum of monomer a), crosslinker b), monomer c)and polymer d). The usage of aqueous monomer suspensions with highsolids contents is also possible.

The monomer solution or the monomer suspension is polymerized on thecontinuous belt forming a polymer gel.

The width of the continuous belt is preferably from 1 to 10 m, morepreferably from 2 to 8 m, most preferably from 3 to 6 m. The length ofthe continuous belt is preferably from 3 to 50 m, more preferably from 5to 40 m, most preferably from 10 to 30 m. The residence time on thecontinuous belt is preferably from 5 to 120 minutes, more preferablyfrom 10 to 60 minutes, most preferably from 12 to 40 minutes.

The materials that are suitable for the continuous belt includesynthetic resins such as polyethylene resin, polypropylene resin,polyester resin, polyamide resin, fluorine resin, polyvinyl chlorideresin, epoxy resin, silicone resin, polystyrene resin, acrylonitrilebutadiene styrene resin, and polyurethane resin, and rubber materialssuch as natural rubber, isoprene rubber, butadiene rubber, styrenebutadiene rubber, chloropropylene rubber, nitrile rubber, nitrileisoprene rubber, acryl rubber, urethane rubber, polysulfide rubber,silicone rubber, and fluorine rubber. Among other substances mentionedabove, rubbers such as nitrile rubber, silicone rubber, andchloropropylene rubber and fluorine resins such aspoly(tetrafluoroethylene), poly(trifluoroethylene),poly(trifluorochloroethylene), and polyvinyl fluoride prove particularlyfavorable. Optional, the belt surface can be coated with surfactants,silicones, waxes and/or water.

The thickness of the formed polymer gel layer is preferably from 1 to 20cm, more preferable from 2 to 15 cm, most preferable from 5 to 10 cm.Next, the polymer gel is comminuted in a further process step, forexample in a meat grinder, extruder or kneader.

The acid groups of the polymer gels obtained are typically in apartially neutralized state, the extent of neutralization preferablybeing in the range from 25 to 95 mol-%, more preferably in the rangefrom 50 to 80 mol-% and even more preferably in the range from 60 to 75mol-%, for which the customary neutralizing agents can be used, forexample alkali metal hydroxides, alkali metal oxides, alkali metalcarbonates or alkali metal bicarbonates and also mixtures thereof.Ammonium salts can also be used instead of alkali metal salts. Sodiumand potassium are particularly preferred as alkali metals, but mostpreference is given to sodium hydroxide, sodium carbonate or sodiumbicarbonate and also mixtures thereof.

Neutralization is preferably carried out at the monomer stage. This iscustomarily accomplished by admixing the neutralizing agent as anaqueous solution, as a melt or else preferably as a solid material. Forexample, sodium hydroxide having a water fraction of distinctly below50% by weight can be present as a waxy mass having a melting point above23 ° C. In this case, metering as piece goods or melt at elevatedtemperature is possible.

Neutralization can also be carried out after polymerization, at thepolymer gel stage. But it is also possible to neutralize up to 40 mol-%,preferably from 10 to 30 mol-% and more preferably from 15 to 25 mol-%of the acid groups before polymerization by adding a portion of theneutralizing agent to the monomer solution and setting the desired finaldegree of neutralization only after polymerization, at the polymer gelstage. When the polymer gel is neutralized at least partly afterpolymerization, the polymer gel is preferably mechanically comminuted,for example by means of a meat grinder, in which case the neutralizingagent can be sprayed, sprinkled or poured on and then carefully mixedin. To this end, the gel mass obtained can be repeatedly grindered forhomogenization.

The polymer gel is then preferably dried with a belt dryer until theresidual moisture content is preferably below 15% by weight andespecially below 10% by weight, the water content being determined byEDANA (European Disposables and Nonwovens Association) recommended testmethod No. WSP 230.2-05 “Moisture content”. Selectively, drying can alsobe carried out using a fluidized bed dryer or a heated plowshare mixer.To obtain particularly white products, it is advantageous to dry thisgel by ensuring rapid removal of the evaporating water. To this end, thedryer temperature must be optimized, the air feed and removal has to bepoliced, and at all times sufficient venting must be ensured. Drying isnaturally all the more simple—and the product all the more white—whenthe solids content of the gel is as high as possible. The solids contentof the gel prior to drying is therefore preferably between 30% and 80%by weight. It is particularly advantageous to vent the dryer withnitrogen or some other non-oxidizing inert gas. Selectively, however,simply just the partial pressure of the oxygen can be lowered duringdrying to prevent oxidative yellowing processes. But in general adequateventing and removal of the water vapor will likewise still lead to anacceptable product. A very short drying time is generally advantageouswith regard to color and product quality.

A further important function of drying the gel is the ongoing reductionin the residual monomer content of the superabsorbent. This is becauseany residual initiator will decompose during drying, leading to anyresidual monomers becoming interpolymerized. In addition, theevaporating amounts of water will entrain any free water-vapor-volatilemonomers still present, such as acrylic acid for example, and thuslikewise lower the residual monomer content of the superabsorbent.

The dried polymer gel is then ground and classified, useful grindingapparatus typically including single or multiple stage roll mills,preferably two or three stage roll mills, pin mills, hammer mills orswing mills.

The polymer obtained may subsequently be postcrosslinked. Usefulpostcrosslinkers are compounds comprising two or more groups capable offorming covalent bonds with the carboxylate groups of the polymers.Useful compounds are for example alkoxysilyl compounds, polyaziridines,polyamines, polyamidoamines, di- or polyglycidyl compounds as describedin EP 83 022 A2, EP 543 303 A1 and EP 937 736 A2, polyhydric alcohols asdescribed in DE 33 14 019 A1, DE 35 23 617 A1 and EP 450 922 A2, orβ-hydroxyalkylamides as described in DE 102 04 938 A1 and U.S. Pat. No.6,239,230. It is also possible to use compounds of mixed functionality,such as glycidol, 3-ethyl-3-oxetanemethanol (trimethylolpropaneoxetane),as described in EP 1 199 327 A2, aminoethanol, diethanolamine,triethanolamine or compounds which develop a further functionality afterthe first reaction, such as ethylene oxide, propylene oxide, isobutyleneoxide, aziridine, azetidine or oxetane.

Useful postcrosslinkers are further said to include by DE 40 20 780 C1cyclic carbonates, by DE 198 07 502 A1 2-oxazolidone and itsderivatives, such as N-(2-hydroxyethyl)-2-oxazolidone, by DE 198 07 992A1 bis- and poly-2-oxazolidinones, by DE 198 54 573 A22-oxotetrahydro-1,3-oxazine and its derivatives, by DE 198 54 574 A1N-acyl-2-oxazolidones, by DE 102 04 937 A1 cyclic ureas, by DE 103 34584 A1 bicyclic amide acetals, by EP 1 199 327 A2 oxetanes and cyclicureas and by WO 2003/31482 A1 morpholine-2,3-dione and its derivatives.

Preferred postcrosslinkers are oxazolidone and its derivatives, inparticular N-(2-hydroxyethyl)-2-oxazolidone, glycidyl compounds, inparticular ethylene glycol diglycidyl ether, polyols, in particularglycerol, and ethylene carbonate.

The amount of postcrosslinker is preferably in the range from 0.001% to5% by weight, more preferably in the range from 0.01% to 2.5% by weightand most preferably in the range from 0.1% to 1% by weight, all based onthe polymer.

Postcrosslinking is customarily carried out by spraying the polymer gelor the dry polymeric particles with a solution, preferably an aqueoussolution, of the postcrosslinker. Spraying is followed by thermaldrying, and the postcrosslinking reaction can take place not only beforebut also during drying.

The postcrosslinker is advantageously mixed with the polymer by theprocess of the present invention and subsequently thermally dried.

Contact dryers are preferable, shovel dryers more preferable and diskdryers most preferable as apparatus in which thermal drying is carriedout. Suitable dryers include for example Bepex® dryers and Nara® dryers.Fluidized bed dryers can be used as well.

Drying can take place in the mixer itself, by heating the shell orblowing warm air into it. It is similarly possible to use a downstreamdryer, for example a tray dryer, a rotary tube oven or a heatable screw.But it is also possible for example to utilize an azeotropicdistillation as a drying process.

Preferred drying temperatures range from 50 to 250° C., preferably from50 to 200° C., and more preferably from 50 to 150° C. The preferredresidence time at this temperature in the reaction mixer or dryer isbelow 30 minutes and more preferably below 10 minutes.

The rotating knifes according to the present invention have an increasedserviceable life. Thus, the present invention provides an improvedprocess for production of superabsorbent polymers with reducedmaintenance costs.

1. A process for production of superabsorbent polymers on a continuousbelt reactor, comprising at least one rotating knife that cuts a formedpolymer gel at an end of the continuous belt reactor, wherein a lengthof a cutting edge is at least 1 cm and the cutting edge is non-parallelto a rotation axis.
 2. The process according to claim 1 wherein a changeof a force that rests on the rotation axis during one revolution is lessthan 20%.
 3. The process according to claim 1 wherein the knife is aspiral knife.
 4. The process according to claim 1 wherein the knife cutsthe polymer gel when the polymer gel moves downward.
 5. The processaccording to claim 1 wherein the cut polymer gel falls into an extruder.6. The process according to claim 1 wherein a difference of a speed ofthe continuous belt and a tip speed of the knife is less than 25%. 7.The process according to claim 6 wherein an end of one knife in rotatingdirection is a beginning of a next knife at an other lateral end of thepolymer gel to be cut.
 8. The process according to claim 1 wherein theknife is mounted on a cylinder.
 9. The process according to claim 8wherein a diameter of the cylinder excluding the knife is from 10 cm to60 cm.
 10. The process according to claim 8 wherein the diameter of thecylinder including the knife is from 20 cm to 70 cm.
 11. The processaccording to claim 8 wherein four spiral knives are mounted on acylinder and each spiral knife forms a ¼ turn spiral.
 12. The processaccording to claim 1 wherein a monomer that is processed on thecontinuous belt reactor is at least 50 wt. % acrylic acid and/or a saltthereof.